Low energy atmospheric muon neutrinos in MACRO

TL;DR

This paper reports measurements of atmospheric muon neutrinos at around 4 GeV energy in MACRO, showing deviations from expectations consistent with muon neutrino oscillations with maximal mixing and specific mass-squared differences.

Contribution

First measurement of low-energy atmospheric muon neutrinos in MACRO supporting neutrino oscillation hypothesis with detailed ratio analysis.

Findings

Observed deficit in muon neutrino events supports oscillation hypothesis.

Data favors muon neutrino oscillations with maximal mixing and Δm² ~ 10^{-3} to 10^{-2} eV².

Results are consistent with higher-energy muon neutrino observations.

Abstract

We present the measurement of two event samples induced by atmospheric $\nu_\mu$ of average energy $ \bar {E}_\nu \sim 4 GeV$. In the first sample, the neutrino interacts inside the MACRO detector producing an upward-going muon leaving the apparatus. The ratio of the number of observed to expected events is $ 0.57 \pm0.05_{stat} \pm0.06_{syst} \pm0.14_{theor}$ with an angular distribution similar to that expected from the Bartol atmospheric neutrino flux. The second is a mixed sample of internally produced downward-going muons and externally produced upward-going muons stopping inside the detector. These two subsamples are selected by topological criteria; the lack of timing information makes it impossible to distinguish stopping from downgoing muons. The ratio of the number of observed to expected events is $0.71 \pm 0.05_{stat} \pm0.07_{syst} \pm0.18_{theor}$ . Using the ratio of the two subsamples (for which most theoretical uncertainties cancel) we can test the pathlength dependence of the oscillation hypothesis. The probability of agreement with the no-oscillation hypothesis is 5% . The deviations of our observations from the expectations has a preferred interpretation in terms of $\nu_\mu$ oscillations with maximal mixing and $\Delta m^2 \sim 10^{-3} \div 10^{-2} eV^2$. These parameters are in agreement with our results from upward throughgoing muons, induced by $\nu_\mu$ of much higher energies.